'Mac' in the Irish means 'son'(not 'son of'); 'Ó' means 'descendant', (not 'grandson of').Both words are nouns, therefore the following name(also a noun of course) is put into the genitive case which can change the spelling.To take the name quoted in the previous post the name'Cairbre' means 'of Cairbreach'(ie the genitive case of Cairbreach). A literal translation of Mac Cairbre would be 'son/of Cairbreach', not 'son of/Cairbre'. With family names if the male version is Mac Cáirbre (son of Cairbreach)then the female version is Nic Cháirbre (daughter of Cairbreach, note that in this instance the initial consonant 'C' is aspirated). If the male descendants of Cairbreach are Ó Cáirbre then the female descendants are Ní Cháirbre.